Reinforcement materials, suitable for the constitution of composite parts

ABSTRACT

The present invention concerns a new intermediate material to be associated with a thermosetting resin for making composite parts, consisting of a unidirectional layer of carbon fibers with a weight of 100 to 280 g/m 2 , associated on each of its faces, with a web of thermoplastic fibers having a thickness of 0.5 to 50 microns, preferably 3 to 35 microns, the intermediate material having a total thickness of 80 to 380 microns, preferably from 90 to 320 microns, and a process for manufacturing composite parts from such a material and the resulting composite parts.

The invention concerns the technical field of reinforcement materialsadapted to the creation of composite parts. More specifically, theinvention concerns a new intermediate material containing aunidirectional layer for fabricating composite parts, by subsequentinjection or infusion of thermosetting resin, a fabrication process forcomposite parts from a stack of such a material, as well as the obtainedcomposite parts.

The manufacture of composite parts or goods, that is, comprising, on onehand, one or more reinforcements or fibrous layers, and on the otherhand, a primarily thermosetting (“resin”) matrix and that can includethermoplastics, can for instance, be achieved by a process called“direct” or “LCM” (from the English “Liquid Composite Moulding”). Adirect process is defined by the fact that one or more fibrousreinforcements are implemented in a “dry” state (that is, without thefinal matrix), the resin or matrix being implemented separately, forinstance by injection into the mold containing the fibrousreinforcements (“RTM” process, from the English Resin TransferMoulding), by infusion through the thickness of the fibrousreinforcements (“LRI” process, from the English “Liquid Resin Infusionor “RFI” process, from the English “Resin Film Infusion”), oralternatively by manual coating/impregnation by roller or brush on eachunit layer of fibrous reinforcement, applied successively on the mold.

For the RTM, LRI or RFI processes, it is generally first necessary tobuild a fibrous preform of the mold of the desired finished product,then to impregnate this preform with a resin. The resin is injected orinfused by differential pressure at temperature, then once all theamount of necessary resin is contained in the preform, the assembly isbrought to a higher temperature to complete thepolymerization/reticulation cycle and thus harden it.

The composite parts used in the automobile, aviation or naval industry,are particularly subject to very strict requirements, notably in termsof their mechanical properties. Indeed, the mechanical properties of theparts are mainly related to a parameter which is the fiber volume ratio(VFR).

In these sectors, a large number of preforms are fabricated based on areinforcement material, primarily carbon fibers, notably of theunidirectional type. It is possible to theoretically calculate themaximum fiber volume ratio contained in a unidirectional layer byassuming two types of structure: hexagonal or square. Assumingrespectively a hexagonal-type structure and a square-type structure, themaximum VFR obtained is respectively 90.7% and 78.5% (An Introduction toComposite Materials, D. Hull, T. W. Clyne, Second Edition, CambridgeSolid State Science Series, 1996). In reality however, it appearsdifficult to obtain fiber volume fractions greater than 70% forcomposite parts. In practice, it is commonly accepted by the personskilled in the art, that a fiber volume ratio (VFR) of about 60% isstandard for implementing satisfactory composite parts, along with goodreproducibility (S. T. Peters, “Introduction, composite basics and roadmap”, in Handbook of Composites, Chapman & Hall, 1998, p. 1-20 andparticularly p. 8).

The resin that is subsequently associated, notably by injection orinfusion, to the unidirectional reinforcement layers during the creationof the part, can be a thermosetting resin, of the epoxy type forinstance. In order to allow a correct flow through a preform composed ofa stack of different layers of carbon fibers, this resin is most oftenvery fluid. The major drawback of this type of resin is its fragilityafter polymerization/reticulation, which results in poor impactresistance of the fabricated composite parts.

In order to solve this problem, the prior art documents proposed theassociation of the unidirectional layers of carbon fibers with a web ofthermoplastic fibers. Solutions such as these are notably described inthe patent applications or patents EP 1125728, U.S. Pat. No. 628,016, WO2007/015706, WO 2006/121961 and U.S. Pat. No. 6,503,856. The addition ofthis web makes it possible to improve mechanical properties in thecompression after impact (CAI) test commonly used to characterize theimpact resistance of the structures.

Document US 2006/0154545 describes such a solution in the case of aunidirectional fabric, but which given the characteristics of thematerial described, does not make it possible to obtain a satisfactoryVFR.

Some details on these prior solutions for unidirectionals are providedbelow. Patent application EP 1125728 in the name of Toray IndustriesInc. describes a reinforcement material that associates a layer ofreinforcement fibers to a short-fiber nonwoven material. The nonwoven islaminated on at least one face of the reinforcement layer, so that thefibers composing the nonwoven pass through the reinforcement fibers (ofcarbon) of the layer and are thus integrated into the reinforcementfibers. The nonwoven consists of a mix of low-melting point fibers andhigh-melting point fibers. It is important to note that all the citedexamples use a single nonwoven material associated on only one face ofthe layer of reinforcement fibers consisting of a fabric or of aunidirectional layer, leading to a non-symmetrical reinforcementmaterial. Example 4 uses a layer of reinforcement fibers consisting of aunidirectional fabric of 300 g/m². The thickness of the nonwoven beingused is not indicated, but it is certainly rather high, given itssurface density (8 g/m²) and its indicated 90% void ratio. The stackused is of the type [−45/0/+45/90]_(2s), that is, 7 interpliescontaining a single nonwoven material. If the instructions in thisdocument are applied to a layer of carbon fibers with a lesser surfacedensity, 134 g/m² for instance, the association with a same type of web,but on each side to obtain a symmetric material, would lead to a verylow fiber volume ratio, non-compatible with the creation of primarystructures for the aeronautic industry.

Patent application WO 2007/015706 in the name of The Boeing Companydescribes a method for the fabrication of preforms combining a stitchedassembly that alternates layers of carbon fibers and layers of nonwovenmaterials to increase the impact resistance of composite structures. Thenonwovens are placed at each interply and not on each side of the carbonfiber layers. This patent application does not mention any range ofsurface density for the carbon layers, nor a range of thicknesses forthe nonwoven materials. The examples mention the use of three differentnonwovens for which only the surface densities of 4.25 g/m² (0.125oz/yd² in American units), 8.5 g/m² (0.25 oz/yd²), and 12.7 g/m² (0.375oz/yd²) are specified. No indication is provided for the thickness ofthese products. One of the webs based on a copolyester actually has anegative effect on the impact resistance properties. The examplesindicate the thickness of the created panels, the surface density of thecarbon layers (190 g/m²) and the type of carbon fibers (T700 with avolume density of 1780 kg/m³). The thicknesses vary from 0.177 to 0.187inch (4.5 to 4.75 mm) for the panels with the best rupture stressresults in compression after impact (CAI). From these thicknesses andthe information about the type of fibers and the surface density of thecarbon plies, it is possible to evaluate the VFR of the panels, whichvaries between 54 and 57%, lower than the value generally considered bythe person skilled in the art for the fabrication of primary parts. Thebest CAI result (39.6 ksi or 273 MPa) is obtained for a VFR of 54%.

In patent application WO 2006/121961, a nonwoven material consisting ofsoluble fibers (of epoxy resins for instance) is intercalated at eachinterply of carbon fiber layers during the creation of the preform. Thenonwoven is not directly associated with the carbon layer. The presentedexample uses a carbon fiber fabric with a surface density of 370 g/m²with a nonwoven material of 60 g/m². The fabricated plate makes itpossible to obtain a VFR of only 55%. At the same time, the lack ofprecision about the compression after impact (CAI) test (nospecification of the impact energy) does not make it possible to deducethe mechanical performances of the indicated measured value.

U.S. Pat. No. 6,503,856 mentions the use of a carbon layer on which twoadhesive layers in the form of webs are superimposed on at least oneside of the carbon layer. This patent does not indicate the thicknessesof the adhesive layers (only the diameters of the fibers of the twolayers) and the preferred surface density of the carbon ranges from 200to 1000 g/m². Sources of electricity (batteries, fuel cells) are thetarget application for this type of product, and the relevance of such aproduct is not highlighted.

Consequently, it appears that the addition of a web to the techniques ofprevious art is carried out most often to the detriment of othermechanical properties. Indeed, as mentioned earlier, the mechanicalproperties are primarily determined by the volume fiber ratio (VFR) andthe techniques described in previous art do not notably make it possibleto obtain composite parts that have a VFR of the order of 60%.

Thus, one of the objectives of the invention is to propose a newintermediate product, adapted to the fabrication of composite partsbased on thermosetting resins, and notably by resin injection orinfusion, that makes it possible to obtain composite parts with a volumefiber ratio of the order of 60% and with satisfactory mechanicalproperties, to meet certain very strict specifications, imposed forinstance in the field of aviation.

Another objective of the invention is to fulfill these specifications,while proposing a symmetrical intermediate product that would be easierto implement and more adapted to automated processes.

In this context, the invention concerns a new intermediate material forthe production of composite parts, by subsequent injection or infusionof a thermosetting resin, consisting of a unidirectional layer of carbonfibers with a surface density of 100 to 280 g/m², associated on each ofits faces with a web of thermoplastic fibers with a thickness of 0.5 to50 microns, the intermediate product according to the invention having atotal thickness in the range of 80 to 380 microns, preferably in therange of 90 to 320 microns.

The invention also concerns a method of fabricating such an intermediatematerial in which a unidirectional layer of carbon fibers with a surfacedensity of 100 to 280 g/m², is associated on each of its faces with aweb of thermoplastic fibers, said webs each having a thickness of 0.5 to50 microns, preferably 3 to 35 microns, through a stage ofmelting/cooling of the webs, such that the intermediate materialpresents a total thickness of 80 to 380 microns, preferably 90 to 320microns.

In another one of its aspects, the invention concerns a stack ofintermediate materials according to the invention, joined togetherwithin the stack. According to a preferred embodiment, such a stack isnot secured by stitching or knitting, but by a weld performed through anoperation of heating/cooling of the web.

Another object of the invention is a fabrication process for a compositepart consisting of the following steps:

-   -   a) create a stack of intermediate materials according to the        invention,    -   b) subsequently consolidate the stack in the form of a preform,    -   c) add a thermosetting resin by infusion of injection,    -   d) consolidate the desired part by a stage of thermal treatment        under pressure, followed by cooling.        -   such that the obtained composite parts notably have a fiber            volume ratio (VFR) of 57 to 63%, preferably of 59 to 61%. In            one particular embodiment of the process according to the            invention, the thermosetting resin is added by infusion at a            pressure lower than atmospheric pressure, notably at a            pressure lower than 1 bar, for example, between 0.1 and 1            bar.

The intermediate material and the process according to the inventionmake it possible to create composite parts with a VFR of the order of60%, which corresponds to the standard factor for primary structures inaviation (that is, the vital parts of the aircraft) and also tosignificantly improve the low-speed impact resistance of the obtainedcomposite parts: for instance, the fall of a tool in a workshop duringthe fabrication of a composite structure, a collision with a foreignbody during its use.

The pressure applied during an injection process is higher than thatused during an infusion process. The result is that it is easier tofabricate parts with a correct VFR, using an injection rather than aninfusion process. The materials according to the invention make itpossible to reach a desired volume fiber ratio, notably of the order of60%, even when the composite part is fabricated with a stage c) whichuses an infusion rather than an injection of resin. Such an embodimentis also an advantageous variant.

The composite parts that can be obtained according to the inventionprocess are also integral parts of the invention, particularly the partswith a volume fiber ratio of 57 to 63% and notably of 59 to 61%.

The following description, with reference to the appended figures, makesit possible to better understand the invention.

FIG. 1 is a cross-section of an intermediate material according to theinvention.

FIG. 2 is a schematic diagram of a machine for fabricating anintermediate material according to the invention.

FIGS. 3A and 3B represent a device for measuring the thickness of apreform under vacuum.

FIGS. 4A and 4B are schematic top views of an intermediate materialaccording to the invention, in which perforations were performed.

FIG. 5 is a partial perspective of a perforation device.

FIG. 6 is a partial view of a device incorporating a variety of meansfor in-line perforation.

FIGS. 7 to 10 are microscopic cross-sections of intermediate productscomposed of a unidirectional layer, associated on each of their largefaces with a (nonwoven) web.

FIGS. 11 through 14 are photographs of the top of stacks welded andperforated according to the invention, FIG. 15 being a photograph fromabove of an intermediate product perforated according to the invention.

FIG. 16 shows the permeability values as a function of the volume fiberratio obtained in several configurations.

FIG. 17 shows the results of mechanical tests.

A “unidirectional layer of carbon fibers” means a layer composedexclusively or quasi-exclusively of carbon fibers placed parallel to oneanother. It is possible to include the presence of thermoplastic bondingstrands, notably, polyamides, copolyamides, polyesters, copolyesters,ester/ether block copolyamides, polyacetals, polyolefins, thermoplasticpolyurethanes, phenoxy, to facilitate the manipulation, if need be, ofthe layer before its association with the thermoplastic fiber webs.These bonding strands will most often lie transversally to the carbonfibers. The term “unidirectional layer” also includes the unidirectionalfabrics in which spaced weft strands cross, by interlacing, the carbonfibers that lie parallel to one another and constitute the warp strandsof the unidirectional fabric. Even in these different cases, where suchbonding, stitching or weft strands are present, the carbon fibersparallel to one another will represent at least 95% of the weight of thelayer, which is therefore qualified as “unidirectional”. However,according to a particular embodiment of the invention, theunidirectional layer contains no weft fiber to interlace with carbonfibers, so as to avoid any undulation. In particular, the intermediatematerial according to the invention contains neither weaving, norstitching, nor knitting. In the unidirectional layer, carbon strands arepreferably, not associated with a polymeric binder and are thereforequalified as dry, meaning that they are neither impregnated, nor coated,nor associated with any polymeric binder before their association withthe thermoplastic webs. Carbon fibers are, however, most oftencharacterized by a high weight ratio of standard sizing that canrepresent at most 2% of their weight.

As part of the invention, the layer of carbon fibers that constitutesthe core of the intermediate material has grammage of 100 to 280 g/m².This range of grammage makes it easy for design engineers to correctlydimension composite structures by adapting the stacking sequences ofdifferent layers, as a function of the different modes of mechanicalstress of the composite structures. A lower carbon grammage of anelementary layer will offer that much more versatility in the choice ofdifferent possible stacks with constant thickness.

The grammage of the unidirectional layer within the intermediatematerial matches that of the unidirectional layer before its associationwith the webs, but it is not possible to measure the grammage of theunidirectional layer before its association with the webs because thestrands have no cohesion with each other. The grammage of the carbonfiber layer can be determined from the grammage of the intermediatematerial (unidirectional layer+2 webs). If the surface density of thewebs is known, it is then possible to deduce the surface density of theunidirectional layer. The surface density is usefully determined fromthe intermediate product by chemical etching (possibly also bypyrolysis) of the web. This type of method is conventionally used by theperson skilled in the art to determine the ratio of carbon fibers of afabric or of a composite structure.

What follows describes a method for measuring the grammage of theintermediate material. The grammage of the intermediate material ismeasured by weighing cut samples of 100 cm² (that is, 113 mm indiameter). To facilitate the cutting of the flexible intermediatematerial samples, the intermediate material is placed between two glossycards from the company Cartonnage Roset (Saint Julien en Genevois,France) of 447 g/m² and a thickness of 0.450 mm to assure a certainrigidity for the assembly. A pneumatic circular punch from the companyNovi Profibre (Eybens, France) is used to cut out the assembly; 10samples are collected for each type of fabricated intermediate product.

In the unidirectional layer, the carbon fibers are most often found inthe form of strands of at least 1000 filaments and notably 3000 to50,000 filaments, for instance 3K, 6K, 12K or 24K. The carbon fibershave a count between 60 and 3800 tex, and preferentially between 400 and900 tex. The thickness of the unidirectional carbon layer varies between90 and 270 μm.

The unidirectional layer is associated on each of its faces with a webof thermoplastic fibers, leading to an intermediate product such asshown in FIG. 1. The use of a symmetric intermediate product makes itpossible to avoid any stacking error during its manual or automaticplacement for the creation of composite parts, and therefore to limitthe generation of fragile zones, notably of an interply without web.

“Web” means a nonwoven material of continuous or short fibers. Inparticular, the fibers composing the nonwoven will have diameters in therange of 0.5 et 70 μm. In the case of a short fiber nonwoven material,the fibers will have a length of 1 to 100 mm.

As part of the invention, the fibers constituting the web areadvantageously made of a thermoplastic material, especially selectedfrom: polyamides (PA: PA6, PA12, PA11, PA6,6, PA 6,10, PA 6,12, . . . ),copolyamides (COPA), the polyamides—block ether or ester (PEBAX, PEBA)polyphthalamide (PPA), polyesters (polyethylene terephthalate -PET-,polybutylene terephthalate -PBT- . . . ), copolyesters (CoPE),thermoplastic polyurethanes (TPU), polyacetals (POM . . . ), polyolefins(PP, HDPE, LDPE, LLDPE . . . ), polyethersulfones (PES), polysulfones(PSU . . . ), polyphenylene sulfones (PPSU . . . ), polyetheretherketone(PEEK), polyetherketoneketone (PEKK), poly (phenylene sulfide) (PPS), orPolyetherimides (PEI), thermoplastics polyimides, liquid crystalpolymers (LCP), phenoxys, block copolymers such asstyrene-butadiene-methylmethacrylate (SBM), methylmethacrylate-butylacrylate-methylmethacrylate copolymers (MAM) or a mixture of fibersconsisting of these thermoplastic materials. The material is of courseadapted to the different types of thermosetting systems used to createthe matrix during the subsequent fabrication of composite parts.

The thickness of the webs before their association with theunidirectional layer will be selected depending on the manner in whichthey will be associated with the layer of carbon fibers. Most often,their thickness will be very close to the thickness desired for theintermediate product. It can also be possible to select the use of athicker web which will be laminated under temperature during theassociation stage so as to reach the desired thickness. In a preferredmanner, the carbon layer is associated on each of its large faces withtwo essentially identical webs so as to obtain a perfectly symmetricalintermediate product. The thickness of the web before association withthe carbon unidirectional layer varies between 0.5 and 200 μm, andpreferably between 10 and 170 μm. On the intermediate product accordingto the invention, the thickness of each web is in the range of 0.5 to 50microns, and preferably in the range of 3 to 35 microns.

The thickness of the different webs before association is determined bythe standard NF EN ISO 9073-2, using method A with a test area of 2827mm² (60 mm diameter disc) and an applied pressure of 0.5 kPa.

The intermediate product of the invention has a thickness in the rangeof 80 to 380 microns, preferably in the range of 90 to 320 microns,which notably makes it possible to achieve the desired fiber volumeratio on the produced final part, even if the latter is fabricated byinfusion under reduced pressure.

The standard NF EN ISO 9073-2 does not allow the measurement of one ofthe components of a material combined from several components. Twomethods have thus been implemented: one to measure the thickness of theweb once it is laminated on the unidirectional layer, and the other tomeasure the thickness of the intermediate product.

Accordingly, the thickness of the nonwoven material or web attached tothe unidirectional carbon layer has been determined from microscopiccross-sections that allow a precision of +/−1 μm. The method is asfollows: An intermediate material associating a unidirectional layercomposed of carbon strands with two webs laminated on each side of thelayer is impregnated using a brush, with a resin which polymerizes atambient temperature (Araldite and Araldur 5052 from the companyHuntsman). The assembly is placed between two plates to apply a pressureof the order of 2−5 kPa during the polymerization. The thicknessmeasurement of the web present in the intermediate product isindependent of the pressure exerted during this step. A slice of theassembly is encased in a cold-casting resin Epofix Kit from Struers,then polished (with a 320 μm silicon carbide abrasive paper anddifferent felts down to 0.3 μm) for viewing with an Olympus BX 60optical microscope combined with an Olympus ColorView IIIu camera. Theimplementation of this resin which polymerizes at ambient temperaturehas no influence on the thickness of the web, but solely makes itpossible to perform the measurements. The software program analySIS auto5.0 from the company Olympus Soft Imaging Solution GmbH makes itpossible to take photographs and obtain the thickness measurements. Foreach intermediate material (unidirectional layer combined with webs oneach side), 5 images are taken at an enlargement of 20. On each image,15 thickness measurements are taken of the web and their average andstandard deviation are determined.

The thickness of the intermediate product was determined with thefollowing method, whose device is shown in FIGS. 3a and 3b , whichdetermines an average for a stack of intermediate products. In thesefigures, A designates the preform; B the support plate; C the siliconpaper; D the vacuum bagging film; E the vacuum fitting; F the drainingfelt, and G the vacuum port. This method is conventionally used by theperson skilled in the art and enables a global measurement whileminimizing the variability that can exist locally within a givenintermediate product. A preform composed of a stack of differentoriented layers of the intermediate product is placed between two layersof 130 g/m² silicon paper with a thickness of 0.15 mm, sold by thecompany SOPAL, in a vacuum bagging film CAPRAN 518 from the companyAerovac (Aerovac Systemes France, Umeco Composites, 1 rue de la Sausse31240 Saint-Jean, France) and in contact with a drainage felt Airbleed10HA sold by Aerovac. The leak-tightness of the assembly is assured by avacuum fitting SM5130 sold by Aerovac. A vacuum of 0.1 to 0.2 kPa ispulled with a Leybold SV40 B vacuum pump (Leybold Vacuum, Bourg lesValence, France). The thickness of the preform is next measured betweentwo TESA Digico 10 digital comparators after subtracting the thicknessof the vacuum bag and of the silicon papers. 25 measurements are takenon each preform and their average and standard deviation are determined.The thickness of the intermediate product is then determined by dividingthe total thickness of the preform by the number of layers ofsuperimposed intermediate products.

The thickness of the intermediate product usefully presents littlevariability, notably with thickness variations not exceeding standarddeviations of 20 μm, preferably not exceeding 10 μm, as illustratednotably in the examples below.

Additionally, the surface density of the web is usefully in the range of0.2 to 20 g/m².

The association between the unidirectional layer and the webs can beobtained in discontinuous batches, for example only at certain points orareas, but is preferably performed using a bond described as continuous,which extends over the entire surface of the layer. The association ofthe unidirectional layer with the two webs can be obtained by means ofan adhesive layer, selected for instance among epoxy adhesives,polyurethane adhesives, thermosetting glues, adhesives based onpolymerizable monomers, structural acrylic or modified acrylicadhesives, and hot-melt adhesives. But most often, the association isenabled by the tacky nature of the hot webs, for example during athermocompression step which assures a bond between the unidirectionallayer and the webs. This stage causes a softening of the thermoplasticfibers of the web, allowing the unidirectional layer to consolidate withthe webs after cooling. The heating and pressure conditions will beadapted to the material constituting the webs and to their thickness.Most often, a thermocompression stage over the entire surface of theunidirectional layer will be created at a temperature in the range ofT_(fweb)−15° C. and T_(fweb)+60° C. (with T_(fweb) designating thefusion temperature of the web) and under a pressure of 0.1 to 0.6 MPa.It is thus possible to achieve compression ratios of 1 to 10 before andafter association with the web. The lamination stage of the web onto theunidirectional carbon material is equally decisive to correctlydetermine the final thickness of the intermediate product. Indeed,depending on the temperature and pressure conditions, notably during thelamination, it is possible to modify and therefore to adjust thethickness of the web on each side of the intermediate product. It isalso conceivable to associate the unidirectional layer to the web(s)only at some areas by localized heating of the web so as to obtain abond that could be likened to a stitching or knitting spot bond used inprior art to produce multiaxial materials and to bind unidirectionallayers to each other. Heating can be obtained by conventional resistanceheating or by ultrasonic means. Spot welds leading to a globaldiscontinuous weld can be considered. The term “spot” is used as part ofthe description to designate individual welds belonging to a set ofwelds and therefore includes different forms of welds. In the plane ofthe intermediate material, spot welds may notably appear in the form ofdiscontinuous or continuous lines, that is, extending over the fullwidth of the unidirectional layer, of spots of various forms, notablycircular or prismatic, of rings . . . . Adapted means of heating, actingas a punch, are used to perform such spot welds. These spot weldsleading to a global discontinuous weld enable a better drapabilty, forinstance. These spot welds are distributed over the surface of theintermediate material to assure its cohesion and make it possible toassure a bond between the unidirectional layer and the webs. In order toassure the bonding of all the fibers of the unidirectional layer, thewelds may extend in continuous lines across over the entire width of theintermediate material, for example transversally to the unidirectionallayer. It is also possible to implement discontinuous lines or spotwelds offset with respect to each other, such that each strandsystematically encounters a weld, for instance at least one weld every100 mm, preferably at least one weld every 10 mm.

The unidirectional layer can be created directly, on line, before itsassociation with the thermoplastic webs. In this case, the strandsneeded to create the layer are then unwound from coils and placed on aconveyor belt so as to extend parallel to each other, and joinedtogether. A process, such as described in patent EP 0972102 may also beimplemented. It is also possible to use a commercial unidirectionallayer whose cohesion and handling will, for instance, be assured bybonding strands, with a mechanical bond by weaving, or with a chemicalbond using the polymer nature of the bonding strands. In all cases, theunidirectional layer that will be secured to the web(s) will providetotal coverage with an openness factor of 0%. For instance, these layersare sold by SIGMATEX UK Limited, Runcom Cheshire WA7 1TE, United Kingdomas item PW-BUD (for instance, product #PC2780600 200GSM/PW-BUD/T700SC12K 50 C/0600 mm), or by the company OXEON AB, Sweden, as item TEXERO.Once the bond between the webs and the unidirectional layer isfabricated, the intermediate material thus obtained generally has anopenness factor of 0%. The openness factor is determined, for instance,by the method described in the examples below.

Thereafter, once the unidirectional layer is associated with theunidirectional layer(s) web(s), it is possible to change its opennessfactor and thus increase its permeability by creating holes orperforations. In such cases, it may be interesting to produce anopenness factor of 0.1 to 5%, preferably between 1 and 2%, obtained withperforations through the thickness of the intermediate material, forexample transversally to it. Because the intermediate material of theinvention is not woven or stitched, it has a controlled fine thicknesswhich allows an openness factor of about 1% to 2%, while offering thepossibility of a laminate with a VFR of 59 to 61%, notably on the orderof 60% for vacuum infusion.

For example, the perforations are systematic and are preferably locatedalong straight lines extending in one or two directions, notably atintervals of 4 to 15 mm. FIGS. 4A and 4B are schematics of thepositioning of perforations 100 in the plane of the intermediatematerial, relative to the direction f corresponding to the direction ofthe unidirectional layer. The distances between the perforations arepurely illustrative, and different variations may be introduced, thelatter being mentioned since they are used in the examples. The welds,from one parallel line to another, may be aligned as shown in FIG. 4A oroffset, notably by a half-step as shown in FIG. 4B. FIG. 4A shows theperforations produced by an alternative mode in which the perforationsextend along two lines perpendicular to each other, each forming anangle of 45° with the strands of the unidirectional layer, such that theperforations are staggered by a half-step, moving parallel andperpendicularly to the strands of the unidirectional layer. FIG. 4Billustrates another mode, in which the perforations extend along twolines perpendicular to each other, one of them being parallel to thestrands of the unidirectional layer. The perforations present on theintermediate material, for instance, are larger, measured parallel tothe surface of the unidirectional layer, ranging from 1 to 10 mm. Forexample, in the plane of the intermediate material, the perforations maybe circular or more or less elongated in the form of an eye or slot asshown in FIG. 15, in particular. The shape of the perforations isnotably a function of the particular perforation device being used.According to such embodiments, it is thus possible to achieve atransversal permeability, notably 10⁻¹³ m² to 10⁻¹⁴ m² for fiber volumeratios of 57 to 63%. Transverse permeability can be defined by theability of a fluid to pass through a fibrous material. It is measured inm². The values given above, as well as those mentioned in the examplesthat follow, are measured with the equipment and measurement techniquedescribed in the thesis entitled “Problems in the measurement oftransverse permeability of fiber preforms for the fabrication ofcomposite structures”, by Romain Nunez, defended at the Ecole NationaleSupérieure des Mines de Saint Etienne, 16 Oct. 2009, which can beconsulted for further details. The measurement is notably performedwhile monitoring the sample thickness during the test using twoco-cylindrical chambers to reduce the influence of “race-tracking”(passage of fluid near or “along the side” of the material whosepermeability is being measured). The fluid used is water and thepressure is 1 bar +/−0.01 bar.

The perforations can be produced with any suitable perforation device,typically for example, needle, pin or other. Heat is applied around theperforation device, so as to retain the openness after the perforationdevice is withdrawn. The perforation is then achieved by the penetrationof a perforation device and by heating around the perforation deviceresulting in a softening of the web, followed by cooling. This producesa fusion of the web around the perforation device which after coolingforms a kind of eyelet around the perforation. When the perforationdevice is removed, the cooling is instantaneous or quasi-instantaneous,thus allowing the perforation to harden. Preferably, the heating deviceis integrated directly into the perforation device such that theperforation device itself is heated as well. In certain cases, it may beadvantageous for the perforation device 110 to have a shoulder 120 asshown in FIG. 5 against which the intermediate material can stop duringthe perforation. The shoulder itself is heated and heats the webs whileexerting pressure on the assembly to be welded, and this occurring overa larger area surrounding the perforation. The duration of the pressureis, for instance, of 0.1 to 2 seconds, preferentially of 0.5 to 1 s. Itis possible to perforate manually or preferably automatically by meansof perforation devices aligned along the selected perforation lines andspacing. In all cases, the perforations are preferably carried out withvery small changes in the thickness of the intermediate material aroundthe perforation. When only a point bonding is planned between the websand the unidirectional layer, the perforations may be performedsimultaneously with the bonding, the adhesion of the webs around theperforations assuring the bonding of the unidirectional layer/webassembly.

As an example, a machine such as shown in FIG. 2 could used forthermocompression bonding and in the absence of perforations. In thecase illustrated in FIG. 2, the webs are associated to the carbon fiberunidirectional layer just after the latter is produced with a desiredsurface density by an attachment or lamination stage under continuousheat and pressure (thermocompression).

The intermediate product according to the invention offers good handlingdue to the presence of thermoplastic webs laminated on each of the facesof the unidirectional layer. This architecture also allows easy cutting,notably without fraying, along non-parallel, notably transversal oroblique directions, with respect to the fibers of the unidirectionallayer.

A stacking or draping of intermediate materials according to theinvention (also named plies) is used to create composite parts. In thestack thus obtained, the plies are generally placed so as to orient atleast two unidirectional layers of plies in different directions. Allthe unidirectional layers may have different directions or only some ofthem, while the others may have identical directions. The favoredorientations are most often oriented in directions at an angle of 0°,+45° or −45° (also corresponding to) +135°, and +90° with the principalaxis of the part to be created. The principal axis of the part isgenerally the largest axis of the part and 0° coincides with this axis.It is, for instance, possible to create stacks that are quasi-isotropic,symmetric, or oriented by selecting the orientation of the plies.Examples of quasi-isotropic stacking include stacking along the anglesof 45°/0°/135°/90° or 90°/135°/0°/45°. Examples of symmetric stackinginclude the angles of 0°/90°/0°, or 45°/135°/45°. Before adding theresin necessary to create the part, it is possible to consolidate theplies among themselves within the stack, notably by an intermediatepreforming stage at temperature and under vacuum or by bonding atseveral points with each added ply, thus creating a preform. Inparticular, it is possible to consider assembling 2 to 32 plies, notably16 to 24 plies. The numbers of plies most currently used are 8, 16, 24and 32 plies, which could for instance be multiples of the 4-plyquasi-isotropic stacks mentioned above.

Advantageously, the stack is secured neither by stitching, nor byknitting, but by a weld produced thanks to the thermoplastic nature ofthe webs present in the stack. To this end, a heating/cooling operationis performed on the entire surface of the stack, or at least on selectedareas at the surface of the stack. The heating causes the fusion or atleast the softening of the web. Such a bond, using the thermoplasticnature of the web, is advantageous because it makes it possible to avoidall the disadvantages represented by the presence of stitching orknitting fibers, such as notably, the problems of undulation,microcracking, reduced mechanical properties of the composite partsobtained subsequently. It is possible to achieve a bond by discontinuouswelding, as opposed to the continuous welding obtained bythermocompression of the entire surface of the stack. A discontinuousweld has an advantage in terms of energy as well as for the drapeabilityof the stack when fabricating subsequent composite parts. Spot weldsleading to a global discontinuous weld can also be considered. As partof the invention, for each unidirectional layer of each subsequentintermediate material, the area of all spot welds is, for example, 0.1to 40%, preferably 0.5 to 15% of the area of the unidirectional layer(this area being equal to the area of one of the faces of the stack).The term “spot” is used as part of the description to designateindividual welds belonging to a set of welds and therefore includesdifferent forms of welds. In the plane of the stack, that is, parallelto the different webs and unidirectional layers, the spot welds maynotably appear in the form of discontinuous or continuous lines, thatis, extending over the full width of the unidirectional layer, of spotsof various forms, notably circular or prismatic, of rings . . . . Thesespot welds are distributed over the surface of the stack to assure itscohesion and make it possible to assure a bond between theunidirectional layers and the webs through the entire thickness of thestack. Such a bond can notably be transversal. Adapted means of heatingmay be used, notably in the form of one or several heating rods in thecase of bond lines or of heated punches in the case of spot bonds, whosecontact point geometry with the stack will be adapted to the form of thedesired spot bonds. Such heating means may be brought to a temperatureof 190 to 220° C. and pressed on the stack with a pressure of 10 to 50kPa, for 0.1 to 2 s for example, and preferentially from 0.5 to 1 s.Ultrasonic welding means of can also be used. Of course, these valuesare purely illustrative and depend notably on the number of plies andthe thermoplastic materials of the webs. For example, spot welds can beproduced systematically and are preferably located along lines extendingin one or two directions, notably at intervals of 4 to 15 mm. FIGS. 4Aand 4B are schematics of the positioning of perforations 100 in parallelto the surface of the stack, with respect to the direction fcorresponding to the direction 0°. The distances between theperforations are purely illustrative, and different variations may beintroduced, the latter being mentioned since they are used in theexamples. Parallel to the surface of the stack, the welds from oneparallel line to the other may be aligned as shown in FIG. 4A or offset,notably by a half-step, as shown in FIG. 4B. As shown in FIGS. 4A and 4Bfor the perforations of an intermediate material according to theinvention, it is possible for instance, to produce the spot welds by analternative mode in which the spot welds extend in the plane of thestack along two lines perpendicular to each other, each forming an angleof 45° to 0°, such that the spot welds are staggered by a half-step, inthe directions 0° and 90°. Another mode consists for example, inproducing spot welds along two lines perpendicular to each other, one ofthem being parallel to 0°.

It is possible to create the stack by adding each ply one by one, andassuring the bond after each addition of a ply. It is equally possibleto produce the bond in a single step, which presents a definiteindustrial interest. To this end, although the heating means describedabove are perfectly suitable, it also possible to use a means of heatingwhich will penetrate within the stack and pass entirely through it so asto produce direct heating on all the webs in the penetration area,including those located in the center of the stack. In this case,concomitantly with the bonding of plies to each other, perforations areproduced in the stack to create diffusion channels for the resin,extending into the thickness of the stack, most often transversally tothe plies of the stack. In these cases, it may be interesting to achievean openness factor of 0.05 to 3%, preferably between 0.1 and 0.6%. Suchopenness factors make it possible to obtain interesting permeabilitiescomparable or superior to those obtained with conventional stitchedmultiaxials. The perforations present in the stack, for example, arelarger measured parallel to the surface of the plies, ranging from 1 to10 mm. According to such embodiment variants, it is thus possible toachieve a transverse permeability for the stack, notably of 10⁻¹¹ m² to10⁻¹⁴ m², preferably 10⁻¹² m² to 10⁻¹³ m² for a VFR of 57 to 63% andnotably for a VFR of 60%. The perforations can be produced with anysuitable perforation device, typically for example, needle, pin orother. Heat is applied around the perforation device, so as to obtainthe desired bond between the plies, which also makes it possible toharden the perforation. As in the case of the perforation of a singleply described above, a fusion of the web occurs around the perforatingdevice, which after cooling leads to a kind or eyelet around theperforation. When the perforation device is removed, the cooling isinstantaneous or quasi-instantaneous, thus allowing the perforation toharden. Preferably, the heating device is integrated directly into theperforation device such that the perforation device itself is heated aswell. It is advantageous for the perforation device to have a shoulderas shown FIG. 5, against which the stack will stop during theperforation, which will help tighten the plies against each other duringthe bond. The shoulder itself is heated and heats the webs whilepressing on the assembly to be welded, and over a larger areasurrounding the perforation. Preferably, the pressure exerted is in therange of 10 to 50 kPa and is selected so as to maintain an essentiallyconstant thickness at all points of the stack. It is possible topuncture manually or preferably automatically by means of perforationdevices 110 aligned along the selected perforation lines and spacing, asshown for instance in FIG. 6.

For the fabrication of composite parts, a thermosetting resin or matrixis then added, for instance by injection, into the mold containing theplies (“RTM” process, from the English Resin Transfer Moulding), or byinfusion (through the thickness of the plies: “LRI” process from theEnglish Liquid Resin Infusion, or “RFI” process from the English ResinFilm Infusion). According to a non-preferred variant, it is equallypossible before building the stack, to perform a manualcoating/impregnation by roller or brush, on each of the plies, appliedsuccessively on the form of the mold being used.

The matrix being used is thermosetting. The injected resin will beselected for instance among the following thermosetting polymers:epoxys, unsaturated polyesters, vinylesters, phenolics, polyimides, andbismaleimides.

The composite part is then obtained after a thermal treatment stage. Inparticular, the composite part is generally obtained by a conventionalhardening cycle of the polymers being used, by performing a thermaltreatment recommended by the polymer suppliers and known by the personskilled in the art. This hardening stage of the desired part is achievedby polymerization/reticulation according to a defined cycle oftemperature and pressure, followed by cooling. The pressure appliedduring the treatment cycle is low in the case of infusion under vacuum,and higher in the case of injection into an RTM mold.

The unperforated stacks according to the invention, even if they areentirely satisfactory for the production of composite parts by injectioninto a mold, in the case of an infusion under reduced pressure, theirapplication is limited to the production of thin parts, for instance ofless than 10 mm. The presence of perforations makes it possible toincrease the permeability of the stack and reach a satisfactory VFR evenon thick parts.

The stack binding modes defined above with spot bonds, with or withoutperforations, can also be implemented with any type of intermediatematerials, intended to be associated with a thermosetting resin for thefabrication of composite parts, which consist of a unidirectional carbonfiber associated on each of its faces with a web of thermoplastic fibersand particularly with intermediate materials other than those defined inthe claims of this patent application. Indeed, whatever webs andunidirectional layers are used, such stacks are interesting in terms ofdrapability and permeability in the case of perforated stacks.Preferably of course, the intermediate materials comply in terms ofthickness and grammage to those described in the invention, since theyachieve high values of VFR in vacuum infusion.

According to a useful characteristic of the invention, the compositeparts obtained have a volume fiber ratio of 57 to 63% and preferably of59 to 61% and notably large thickness, notably larger than 10 mm. Thesevolume fiber ratios are compatible with the use of structures forprimary parts, that is, critical parts in aviation that withstandmechanical stress (fuselage, wings . . . ).

The volume fiber ratio (VFR) of a composite part is calculated from ameasurement of the thickness of a composite part, knowing the surfacedensity of the unidirectional carbon layer and the properties of thecarbon fiber, using the following equation:

$\begin{matrix}{{{TVF}(\%)} = {\frac{n_{plis} \times {Masse}\mspace{14mu}{surfacique}\mspace{14mu}{UD}_{carbone}}{\rho_{{fibre}\mspace{14mu}{carbone}} \times e_{plaque}} \times 10^{- 1}}} & (1)\end{matrix}$

Where e_(plaque) is the thickness of the plate in mm,

-   -   ρ_(fibre carbone) is the density of the carbon fiber in g/cm³,    -   the surface density of the carbon UD_(carbone) is in g/m².

The composite parts obtained also have optimum mechanical properties,notably impact resistance (CAI, Compression After Impact), thesemechanical properties showing the sensitivity to holes such as open holecompression (OHC, Open Hole Compression in English), open hole tension(OHT, Open Hole Traction English), bearing (Bearing in English),in-plane shear (IPS, In-Plane Shear in English). In particular, it ispossible to obtain composite parts with stress rupture in compressionafter impact (CAI), measured according to the preliminary Europeanstandard prEN 6038 published by ASD-STAN (AeroSpace and DefenceStandard, Avenue de Tervuren 270, 1150 Woluwe-Saint-Pierre, Belgium),greater than 200 MPa under an impact of 25 J. Also noted, especiallywhen the resin matrix is of epoxy, was a small decrease of the epoxy Tgafter aging, of the same order of magnitude as that obtained forstandard preimpregnates, known by the person skilled in the art.

The following examples illustrate the invention, but are not limiting innature.

1.1. Materials Used.

The intermediate products tested are unidirectional layers composed ofcarbon fibers associated with a web on each side. Three types of carbonfibers have been used: 12K intermediary module (IM) fibers sold byHexcel, 12K high resistance (HR) fibers sold by Hexcel, 12K highresistance (HR) fibers sold by Toray; their mechanical and physicalproperties are shown in Table 1.

Several carbon surface densities of the unidirectional layers weretested. These layers are fabricated on line and their carbon fibergrammage is estimated at 134 g/m²±3% for the Hexcel IM carbon fibers,194 g/m²±3% for the Hexcel IM fibers, 134 g/m²±3% for the Hexcel HRfibers, 268 g/m²±3% for the Hexcel HR fibers and 150 g/m²±3% for theToray HR fibers.

TABLE 2 Characteristic properties of the carbon fibers Hexcel Hexcel IMHR Toray HR Stress rupture (MPa) 5610 4830 4900 Tensile modulus (GPa)297 241 240 Elongation (%) 1.9 1.8 2 Weight/unit length (g/m) 0.4430.785 0.800 Volume density (g/cm³) 1.80 1.79 1.80 Filament diameter (μm)5 7 7

Three types of webs were used, named web 1, web 2, (1R8D03 sold byProtechnic, 66, rue des Fabriques, 68702—CERNAY Cedex—France), web 3.These webs are based on a mix of polyamides and copolyamides (web 1 and2) or on polyamides (web 3). This type of web is also sold by suchcompanies as Spunfab Ltd./Keuchel Associates, Inc. (175 Muffin LaneCuyahoga Falls, Ohio 44223, USA). Web 1 is composed of continuousfilaments. Webs 2 and 3 are composed of short fibers.

The characteristics of the webs used are indicated in Table 3. Themelting point of the webs shown in Table 2 is determined by differentialsweep calorimetry (DSC) according to the ISO 11357-3 standard. Thesurface density is measured according to the ISO 3801 standard. Theporosity factor shown in Table 2 is calculated with the followingformula:

$\begin{matrix}{{{Taux}\mspace{14mu}{de}\mspace{14mu}{{{porosit}é}_{voile}(\%)}} = {1 - {\frac{{Masse}\mspace{14mu}{surfacique}\mspace{14mu}{du}\mspace{14mu}{voile}}{\rho_{{matière}\mspace{11mu}{du}\mspace{11mu}{voile}} \times e_{voile}} \times 100}}} & (2)\end{matrix}$

Where—the surface density of the web is expressed in kg/m²,

-   -   ρ_(matière du voile) density of the web material, is expressed        in kg/m3    -   e_(voile) is expressed in m.

TABLE 3 Characteristics of the webs used (the values indicated after ±represent the standard deviation) Reference 1 2 3 Melting point of theweb (° C.) 178 160 178 Surface density (g/m²) 6.7 ± 0.5 2.8 ± 0.1 3.7 ±0.1 Filament diameter (μm)* 44 ± 12 9 ± 2 13 ± 3± Thickness of the web(μm) 161 ± 18  59 ± 12 69 ± 12 Porosity factor (%) calculated  96  98 97 with formula (2) *Measured by image analysis

2. Fabrication of the Tested Intermediate Products

The web is laminated directly on each side of the unidirectional layersbased on carbon fibers using a machine (FIG. 2) specifically dedicatedfor this purpose, just after the formation of the layer with the desiredgrammage. The carbon strands 1 are unrolled from carbon spools 3 mountedon a creel 4, pass through a comb 5, are brought into the axis of themachine by a guide roller 6 and a comb 7, and a guide bar 8 a. Thecarbon strands are preheated with a heating rod 9 and are then spread bya spreading bar 8 b and the heating rod 10 to the desired carbon surfacedensity of the unidirectional layer 17. The web coils 13 a and 13 b areunwound without tension and transported with the moving belts 15 a and15 b attached between the free rotation rollers 14 a, 14 b, 14 c, 14 dand the heating rods 12 a, 12 b. Webs 2 a and 2 b are preheated in zones11 a and 11 b before coming into contact with the carbon strands 1, andpasted on each side of two heating rods 12 a and 12 b whose air gap iscontrolled. A coolable calendar 16 then applies pressure on theunidirectional layer with a web on each side 17. A return roller 18redirects the product 17 toward the tensioning system comprising threedraw rollers 19 then winding rollers 20 driven by a motor to form a coilcomposed of the claimed intermediate product 17.

The test conditions for the fabrication of the carbon unidirectionallayers combined with a web on each side are shown in Table 3 below.

TABLE 3 Process parameters for the fabrication of unidirectional layersassociated with a web on each side Measured T _(web preheat web) T_(bar) surface density Line T _(bar) T bar (° C.) (° C.) of intermediateWeb speed (° C.) (° C.) (11a & (12a & Example Fiber Type product (g/m²)type (m/min) (9) (10) 11b) 12b) Comparative Hexcel IM 134 No web — — — ——  1  2 Hexcel IM 149 Web 1 1.3 200 200 120 270 Comparative Hexcel IM149 Web 1 1.3 200 200 120 170  2b  3 Hexcel IM 141 Web 2 1.6 200 200 120255  3b Hexcel IM 141 Web 2 1.3 200 200 120 143  4 Hexcel IM 142 Web 31.8 200 200 120 265  4b Hexcel IM 142 Web 3 1.3 200 200 120 187Comparative Hexcel IM 199 No web — — — — —  5  6 Hexcel IM 213 Web 1 1.6200 200 120 270  6b Hexcel IM 213 Web 1 1.3 200 200 120 170  7 Hexcel IM197 Web 2 1.8 200 200 120 255  7b Hexcel IM 197 Web 2 1.3 200 200 120145  8 Hexcel IM 207 Web 3 1.3 200 200 120 265  8b Hexcel IM 207 Web 31.3 200 200 120 193 Comparative Toray HR 150 No web — — — — —  9 10Toray HR 168 Web 1 1.3 200 200 120 255 10b Toray HR 168 Web 1 1.3 200200 120 188 11 Toray HR 159 Web 2 1.6 200 200 120 250 11b Toray HR 159Web 2 1.3 200 200 120 190 12 Toray HR 162 Web 3 1.3 200 200 120 265 12bToray HR 162 Web 3 1.3 200 200 120 210 Comparative Hexcel HR 136 No web— — — — — 13 14 Hexcel HR 156 Web 1 1.3 200 200 120 270 14b Hexcel HR157 Web 1 1.3 200 200 120 170 15 Hexcel HR 147 Web 2 1.8 200 200 120 25515b Hexcel HR 146 Web 2 1.3 200 200 120 145 16 Hexcel HR 147 Web 3 1.5200 200 120 265 16b Hexcel HR 150 Web 3 1.3 200 200 120 190 ComparativeHexcel HR 268 No web — — — — — 17 18 Hexcel HR 281 Web 1 1.3 200 200 120270 18b Hexcel HR 281 Web 1 1.3 200 200 120 170 19 Hexcel HR 274 Web 21.6 200 200 120 255 20 Hexcel HR 276 Web 3 1.3 200 200 120 265 20bHexcel HR 276 Web 3 1.3 200 200 120 190

-   -   In the case of unidirectional layers without web (comparative        example 1), the carbon strands are secured with a 280 dtex        hot-melt strand distributed every 50 mm perpendicularly to the        orientation of the carbon fibers. In the case of the        representative examples of the invention, where the        unidirectional layers are associated with two webs, the layers        are formed directly on the machine, before the lamination with        the web.

3. Thickness Determination after Lamination of the Web and theIntermediate Product

The thicknesses of the webs after lamination on the unidirectionallayers are measured by image analysis. Table 4 shows the averagethicknesses and the standard deviations of the webs (for 75 values)obtained by this method for each examined configuration. This same Table4 indicates the thicknesses of the various intermediate productscontaining the carbon layers associated with a web on each side. Thesemeasurement are derived from measurements of preform thicknesses atatmospheric pressure according to the described methods.

TABLE 4 Thickness of intermediate products (UD layers associated with aweb on each side) and of the webs on these layers Standard Thickness ofStandard Thickness of deviation layer (μm) deviation web on layer of web(UD + web) of layer Example (μm) thickness (μm) thickness Comparative  1— — 120 4  2 20 8 153 3 Comparative  2b 62 15  183 4  3 12 6 120 4  3b14 5 123 5  4 13 4 145 4  4b 23 7 157 4 Comparative  5 — — 175 4  6 21 9198 2  6b 32 8 224 3  7  9 3 184 5  7b 11 3 197 3  8 11 3 185 3  8b 20 7195 3 Comparative  9 — — 131 2 10 19 6 169 3 10b 48 16  204 3 11 13 6166 4 11b 13 4 161 4 12 12 3 155 4 12b 22 7 163 4 Comparative 13 — — 1234 14 20 7 162 4 14b 46 12  192 3 15 12 4 155 3 15b 15 6 157 7 16 15 5154 3 16b 21 9 164 6 Comparative 17 — — 237 3 18 19 7 287 5 18b 49 25 301 4 19 12 6 264 4 20 16 6 280 7 20b 22 8 286 6

FIG. 7 is a micrographic cross-section of the intermediate product ofexample 2b (134 g/m² of Hexcel IM carbon fibers associated with web 1 oneach side).

FIG. 8 is a micrographic cross-section of the intermediate product ofexample 2 (134 g/m² of Hexcel IM carbon fibers associated with web 1 oneach side).

FIG. 9 is a micrographic cross-section of the intermediate product ofexample 3b (134 g/m² of Hexcel IM carbon fibers associated with web 2 oneach side).

FIG. 10 is a micrographic cross-section of the intermediate product ofexample 4 (134 g/m² of Hexcel IM carbon fibers associated with web 3 oneach side).

4. Fabricating the Plates

4.1 Definition of the Stack Sequence

The plates obtained are quasi-isotropic, that is, they consist of anassembly of elementary plies with the different orientations)(0°/45°/−45°/90°. The stack is also symmetrical. The number of pliesforming the stack is determined from the following formula, derived fromformula (1):

$\begin{matrix}{n_{plis} = \frac{{{TVF}(\%)} \times \rho_{{fibre}\mspace{14mu}{carbone}} \times e_{plaque}}{{Masse}\mspace{14mu}{surfacique}\mspace{14mu}{UD}_{carbone}}} & (2)\end{matrix}$

where:

-   -   the desired thickness of the plate is the closest to 4 mm        (defined by the standard prEN 6038), e_(plaque) is expressed in        mm,    -   the intended fiber volume ratio (VFR) for the best mechanical        properties, is 60% and ρ_(fibre carbone) fiber is expressed in        g/cm³,    -   the surface density of the UD_(carbone) is expressed in g/m².

The stack thus consists of 32 plies for a carbon grammage of 134 and 150g/m², and is written in brief notation as: [+45/0/−45/90]_(4s). Forcarbon grammages of 194 and 268 g/m², the number of plies is 24 and 16plies respectively. The stack is written in brief notation[+45/0/−45/90]_(3s) and [+45/0/−45/90]_(2s). Each ply corresponds to aweb/UD/web material.

4.2 Fabrication of the Composite Plate

The different plies are secured to each other by lightly welding atseveral points after each addition of a new ply, using a soldering iron.The assembly forms a preform. The 340 mm×340 mm preform created by thestacking sequence adapted to the carbon grammage is placed in aninjection mold in a press. A frame of known thickness surrounds thepreform in order to obtain the desired fiber volume ratio (VFR).

The epoxy resin sold as HexFlow RTM6 by Hexcel is injected at 80° C. at2 bars through the preform, which is maintained at 120° C., thetemperature of the press platens. The pressure applied to each of thetwo press platens is 5 bars. When the resin appears at the exit of themold, the exit tubing is closed and the polymerization cycle begins(increase to 180° C. at 3° C./min, then maintained for 2 hours at 180°C., then cooling at 5° C./min). Six 150×100 mm samples per type ofconfiguration (standard prEN 6038) are then cut to perform thecompression after impact (CAI) test.

5. Mechanical Tests

The samples (6 per type of configuration) were attached to a device asindicated in standard prEN 6038. The samples were subjected to a singleimpact with an energy of 25J using equipment adapted to the preliminaryEuropean standard prEN 6038 published by ASD-STAN (AeroSpace and DefenceStandard, Avenue de Tervueren 270, 1150 Woluwe-Saint-Pierre, Belgium).The compression tests were performed on a 100 kN mechanical test machineInstron 5582 rebuilt by the Zwick company (Zwick France Sari, RoissyCharles de Gaule, France).

The results of compression rupture stress after impact are shown inTables 5a to 5e.

TABLE 5a Results of stress rupture in compression after impact (CAI) of25 J for different types of 134 g/m2 unidirectional IM for differenttypes of webs Comparative Example Comparative Example Example ExampleExample example 1 2 example 2b 3 3b 4 4b CAI (MPa) 142 260 275 289 279305 308 Standard 6 15 17 6 18 24 19 deviation (MPa)

TABLE 5b Results of stress rupture in compression after impact (CAI) of25 J for different types of 194 g/m² unidirectional IM for differenttypes of webs Comparative Example Example Example example 5 Example 6 6bExample 7 7b Example 8 8b CAI (MPa) 126 319 247 285 282 291 294 Standard14 8 4 5 3 17 5 deviation (MPa)

TABLE 5c Results of stress rupture in compression after impact (CAI) of25 J for different types of 150 g/m² unidirectional HR Toray fordifferent types of webs Comparative Example Example Example example 9Example 10 10b Example 11 11b Example 12 12b CAI (MPa) 151 312 337 354294 320 313 Standard 11 9 12 9 21 11 18 deviation (MPa)

TABLE 5d Results of stress rupture in compression after impact (CAI) of25 J for different types of 134 g/m² unidirectional Hexcel HR fordifferent types of webs Comparative Example Example Example example 13Example 14 14b Example 15 15b Example 16 16b CAI (MPa) 175 299 306 280280 288 323 Standard 11 17 17 12 12 11 20 deviation (MPa)

TABLE 5e Results of stress rupture in compression after impact (CAI) of25 J for the 268 g/m² unidirectional Hexcel HR without web and with web2 Compar- ative Example Example Example Example Example Example 17 1818b 19 20 20b CAI 135 288 274 253 248 248 (MPa) Standard 13 16 14 14 139 deviation (MPa)

6. Control of Plate Thickness and Derivation of Volume Fiber Ratios(VFR)

The plates were positioned between two digital comparators TESA Digico10 to measure their thickness. 24 equidistant measurements on thesurface were performed per plate.

Tables 6a to 6e present the results of plates thickness measurementsobtained from different fabricated intermediate materials. Given thethicknesses of the plates, the different VFRs can be calculated fromformula (2). The comparative example 2b shows the influence of thethickness of the webs laminated on the unidirectional layer. Thethickness of the web laminated on the layer in the case of example 2b(Table 4) is 62 μm, which is more than the claimed web thickness. Theuse of this thicker web leads to the fabrication of a part with a volumefiber ratio lower than that required for the use of the part as aprimary structure.

TABLE 6a Thickness measurement of the different plates fabricated fromunidirectional layers of carbon fibers 134 g/m² Hexcel IM with differenttypes of web; stack sequence [+45/0/−45/90]_(4s) Comparative ExampleComparative Example Example Example Example example 1 2 example 2b 3 3b4 4b Measured 3.93 3.94 4.25 3.95 3.90 3.94 3.95 thickness (mm) Standard0.03 0.02 0.2 0.03 0.05 0.02 0.03 deviation Calculated 60.7 60.5 56.060.3 61.2 60.5 60.3 VFR (%)

TABLE 6b Thickness measurement of the different plates fabricated fromunidirectional layers of carbon fibers 194 g/m² Hexcel IM with differenttypes of webs; stack sequence [+45/0/−45/90]_(3s) Comparative ExampleExample Example Example Example Example example 5 6 6b 7 7b 8 8bMeasured 4.28 4.24 4.32 4.25 4.31 4.32 4.28 thickness (mm) Standard 0.060.03 0.02 0.05 0.05 0.03 0.051 deviation Calculated VFR 60.4 61.0 59.960.9 60.0 59.9 60.5 (%)

TABLE 6c Thickness measurement of the different plates fabricated fromunidirectional layers of carbon fibers 150 g/m² HR Toray with differenttypes of webs; stack sequence [+45/0/−45/90]_(4s) Comparative ExampleExample Example Example Example Example example 9 10 10b 11 11b 12 12bMeasured thickness 4.44 4.40 4.45 4.48 4.39 4.4 4.4 (mm) Standarddeviation 0.05 0.03 0.04 0.04 0.05 0.03 0.04 Calculated VFR (%) 60.861.3 60.6 60.2 61.4 61.3 61.3

TABLE 6d Thickness measurement of the different plates fabricated fromunidirectional layers of carbon fibers 134 g/m² Hexcel HR with differenttypes of webs; stack sequence [+45/0/−45/90]_(4s) Comparative ExampleExample Example Example Example Example example 13 14 14b 15 15b 16 16bMeasured 3.9 4.09 4.11 3.96 3.90 3.98 4.09 thickness (mm) Standard 0.050.10 0.10 0.04 0.04 0.03 0.09 deviation Calculated VFR 61.4 58.6 58.360.4 61.4 60.3 58.6 (%)

TABLE 6e Thickness measurement of the different plates fabricated fromunidirectional layers of carbon fibers 268 g/m² Hexcel HR with differenttypes of webs; stack sequence [+45/0/−45/90]_(2s) Comparative ExampleExample Example Example Example example 17 18 18b 19 20 20b Measured3.87 3.98 3.97 3.93 3.92 3.89 thickness (mm) Standard 0.04 0.03 0.040.04 0.04 0.06 deviation Calculated VFR (%) 61.8 60.2 60.8 61 61.2 61.6

Equation (3) makes it possible to calculate the volume fiber ratio ofeach composite plate fabricated by injection. It is important to notethat independently of the configuration used, the VFR of the platesfalls within the range of 60±2%, which is an indispensable criterion forobtaining primary structure parts.

7. Examples with Perforations

A robot equipped with a punch head as illustrated in FIG. 5 was used.Two diameters were used for the penetrating portion of the head: 0.8 mmand 1.6 mm. To perform the welding-penetrations, the heads were heatedto a temperature of 200° C. and the perforations were made with apressure of 30 kPa for 0.8 s.

7.1 Spot Welded Quasi-Isotropic Multiaxial (4 Plies)

A stack of 4 plies, oriented 45°, 0°, 135° and 90° was produced on lineon a multiaxial fabrication machine. Spot welds spaced by 9, asillustrated in FIG. 4A, were created in the orientations 0° and 90° withrespect to the axis of the machine. Alternately, spot welds spaced by4.5 mm and 4.5 mm, as illustrated in FIG. 4B, were created in theorientations +45° and +135° with respect to the axis of the machine. Thefollowing stacks were created:

-   -   Example 21: 4 plies according to example 8—perforation head        ø1.6, a photograph of which is shown in FIG. 11    -   Example 22: 4 plies according to example 8—perforation head ø1.6        in alternating fashion, a photograph of which is shown in FIG.        12    -   Example 23: 4 plies according to example 8—perforation head        ø0.8, a photograph of which is shown in FIG. 13    -   Example 24: 4 plies according to example 8—perforation head        ø0.8, in alternating fashion, a photograph of which is shown in        FIG. 14    -   Example 25: 1 ply according to example 8—perforation head ø1,6,        a photograph of which is shown in FIG. 15    -   Example 26: 4 plies according to example 8 welded (without        perforation)—The welding uses a head with a diameter of 8 mm        heated to a temperature of 200° C., identical to the perforation        heads but without the needle. The welds are placed according to        FIG. 4a , but spaced by 50 mm. The welds are performed with a        pressure of 30 kPa.

And as a comparison for the transverse permeability:

-   -   Twill fabric 2/2—ref. Hexcel 48302    -   Quasi-isotropic multiaxial 4×194 g/m² HR stitched with thread 76        dtex—5 mm×5 mm—chain stitch

7.2 Transverse Permeability Measurements

The machine and measurement method are described in the thesis entitled“Problems in the measurement of transverse permeability of fibrouspreforms for the fabrication of composite structures,” by Romain Nunez,defended at the Ecole Nationale Supérieure des Mines de Saint Etienne,16 Oct. 2009. The FVR variation is obtained by successive variations ofthe sample thickness. Four tests were conducted for each type ofmaterial. The results are presented in FIG. 16 and show that the lowestcurve corresponds to the permeability of four intermediate materials inaccordance with the invention, which have been welded in the absence ofperforation. It clearly appears that the resulting stack is verypermeable, and that it will therefore be difficult to impregnate undervacuum to a great thickness. The perforations can incontestably improvethe permeability to approach or exceed that obtained with a stitchedmultiaxial. The creation of perforations on an intermediate material canalone also significantly improve transverse permeability, but to alesser extent compared with a four-ply complete and perforated stack.

Volume fiber ratio in % Permeability values in m² 55 60 Example 225.5E−13 3.7E−13 Example 21 2.2E−13 1.4E−13 Example 24 4.6E−14 3.5E−14Example 23 4.8E−14 3.8E−14 Twill fabric 2/2 - 4 plies ref. Hexcel 483021.8E−14 1.3E−14 Example 25 9.7E−15 7.4E−15 Example 26 2.4E−15 1.9E−15

7.3 Openness Factor Measurements

The openness factors were measured using the following method.

The device consists of a SONY camera (model SSC-DC58AP), equipped with a10× objective, and a Waldmann light table, model W LP3 NR, 101381 230V50 HZ 2×15 W. The sample to be measured is placed on the light table,the camera is mounted on a frame, positioned at 29 cm from the sample,and then focused.

The width measurement is determined as a function of the fibrousmaterial to be analyzed, using the (zoom) ring and a ruler: 10 cm foropen fibrous materials (OF>2%), 1.17 cm for less open fibrous materials(OF<2%).

Using the diaphragm and a control image, the brightness is adjusted toobtain an OF value corresponding to the one of the control image.

The Videomet contrast measurement program, from the company Scion Image(Scion Corporation, USA) is used. The image obtained is processed asfollows: using a tool, a maximum area is defined, corresponding to thechosen calibration, for example 10 cm—70 perforations, and comprising aninteger number of patterns. An elemental area is then selected, in thetextile meaning of the term, that is, an area that describes thegeometry of the fibrous material as a repeating pattern.

With the light from the light table passing through the openings of thefibrous material, the OF percentage is defined by one hundred from whichis subtracted the black area divided by the elemental area, that is,100−(black area/elemental area).

It should be noted that the brightness control is important becausediffusion phenomena can alter the apparent size of the perforations andtherefore the OF. An intermediate brightness will be chosen, such thatno excessive saturation or diffusion phenomenon will be visible.

The results obtained are shown in TABLE 7 below:

Ex. 21 Ex. 22 Ex. 23 Ex. 24 Ex. 25 Average 0.14 0.54 0.08 0.13 0.96 % OFStandard 0.002 0.15 0.03 0.05 0.06 deviation

It should be noted that the openness factor of a perforated ply israther high (about 1%) and is higher than those obtained with perforatedstacks, while the permeability of the perforated ply alone is lower thanthose of the stacks. It thus seems that a single perforation performedduring the assembly of different plies to form a stack is more efficientin terms of permeability than the stacking of intermediate materialsperforated separately. It is indeed conceivable that the fluid resinpenetrates more easily through channels already created through severalthicknesses. Whatever the case may be, the increased permeability ofeach ply (2·10⁻¹⁵ m² to

7·10⁻¹⁵ m² for a VFR of 60%) related to its increased openness factor(from 0 to 1%) is very important and will make it possible to increasethe possible thicknesses of the laminates created with intermediatematerials according to the invention.

7.4 Mechanical values

Multiaxial stacks welded according to the invention were compared to thesame stitched multiaxials.

The reference stitched multiaxials consist of plies corresponding toexample 12. The stitching thread is a 76 dtex polyamide thread, with a 5mm×5 mm chain stitch.

The welded multiaxials were made with the same plies as for example 12,but welded in a square pattern according to FIG. 4A, but at 50 mm×50 mm,with a spot weld diameter of 8 mm, and therefore without stitching. Thestack used for the tests is[(90/+45/0)/(0/−45/90)/(90/+45/0)/(90/−45/0)]_(s). The standards usedare listed in Table 8 below.

The following specific conditions were used. Rectangular samples of190×25 mm² were used in traction, and a cord modulus between 1000 and6000 μm/m was calculated. Samples of 150×25 mm² with a perforationdiameter of 5 mm were used for open hole traction. Samples of 150×25 mm²with a perforation diameter of 5 mm and a 100° countersink with a depthof 2.1 mm carrying a 5RH8035M ST 39584 nut (0.35 daN·m clamping torque)and a 22258 TX 050 005 ST 38260 screw were used. Samples of 115×25 mm²with 5 mm perforations were used for open hole compression. The head andthe foot of the sample were loaded. In filled-hole compression, samplesof 115×25 mm² with a perforation diameter of 5 mm and a 100° countersinkwith a depth of 2.1 mm carrying a 5RH8035M ST 39584 nut (0.35 daN·mclamping torque) and a 22258 TX 050 005 ST 38260 screw were used. Thehead and the foot of the samples were loaded.

The tests were conducted under laboratory conditions of standardizedhumidity and temperature (“dry”, “room temperature” tests).

TABLE 8 Standard Test Type Batch Average deviation Tension stress (MPa)Stitched 816 29 Standard: EN 6035 type 2 Welded 903 4 Tensile modulus(GPa) Stitched 47.87 0.40 Standard: EN 6035 type 2 with a cord Welded48.52 0.57 modulus of 1000-6000 μm/m Open hole tension stress [MPa]Stitched 489 21 Standard: EN 6035 type 1 Welded 462 21 Filled holetension stress [MPa] Stitched 397 15 Standard: EN 6035 type 1 +countersink Welded 379 24 100° and screw Open hole tension stress [MPa]Stitched 247 7 Standard: EN 6036 type 1 Welded 244 4 Filled hole tensionstress [MPa] Stitched 302 14 Standard: EN 6036 type 3 Welded 291 16Tensile modulus (GPa) Stitched 479 4 Enlargement: ×10 Welded 485 6

FIG. 17 shows all these results.

The invention claimed is:
 1. An intermediate material, intended to beassociated with a thermosetting resin for fabricating composite parts,said intermediate material having a first surface and a second surface,said intermediate material consisting of: a unidirectional layer ofcarbon fibers with a surface density of 100 to 280 g/m² and a thicknessof between 90 and 270 microns, said unidirectional layer of carbonfibers having a first side and a second side; and a first web whichcomprises nonwoven thermoplastic fibers that have diameters of from 0.5microns to 16 microns, said first web having an interior side locatednext to the first side of said unidirectional layer of carbon fibers andan exterior side which forms the first surface of said intermediatematerial, wherein said first web is thermally bonded to the first sideof said unidirectional layer of carbon fibers and wherein the distancebetween the interior side and exterior side of said first web is from 3to 15 microns; a second web which comprises nonwoven thermoplasticfibers that have diameters of from 0.5 microns to 16 microns, saidsecond web having an interior side located next to the second side ofsaid unidirectional layer of carbon fibers and an exterior side whichforms the second surface of said intermediate material, wherein saidsecond web is thermally bonded to the second side of said unidirectionallayer of carbon fibers and wherein the distance between the interiorside and exterior side of said second web is from 3 to 15 microns; and aplurality of surfaces which define a plurality of holes wherein each ofsaid holes has a perimeter and wherein each of said holes extends fromthe first surface of said intermediate material to the second surface ofsaid inter material to thereby form a plurality of perforations throughsaid intermediate material and wherein said thermal bonding of saidfirst and second webs to said layer of unidirectional carbon fibers isprovided only by spot thermal bonding welds that are in the formdiscontinuous lines of spot welds which extend across the first andsecond surfaces said intermediate material.
 2. An intermediate materialaccording to claim 1, wherein the variation in thickness of theintermediate material does not exceed 20 μm in standard deviation.
 3. Anintermediate material according to claim 1 wherein the unidirectionallayer contains no weft thread interlacing with the carbon fibers.
 4. Anintermediate material according to claim 1 wherein the intermediatematerial does not contain weaving, stitching or knitting.
 5. Anintermediate material according to claim 1 wherein the first web andsecond web are essentially identical.
 6. An intermediate materialaccording to claim 1 wherein the first and second webs have a surfacedensity of 2.8±0.1 g/m².
 7. An intermediate material according to claim1 wherein said thermoplastic fibers located in said first and secondwebs have a length of from 1 to 100 mm.
 8. An intermediate materialaccording to claim 1 wherein said first web and said second web have ainciting point of 160° C.
 9. An intermediate material according to claim1 wherein the first and second webs have a surface density of from 0.5g/m² to 3.8 g/m².
 10. An intermediate material according to claim 9wherein the first and second webs have a surface density of 33±0.1 g/m².11. An intermediate material according to claim 1 wherein thethermoplastic fibers located in said first and second webs havediameters of 9±2 microns.
 12. An intermediate material according toclaim 1 wherein the thermoplastic fibers located in said first andsecond webs have diameters of 13±3 microns.
 13. An intermediate materialaccording to claim 1 wherein the number of perforations through saidintermediate material is sufficient to provide an intermediate materialhaving an openness factor of from 0.1 to 5 percent.
 14. An intermediatematerial according to claim 1 where said spot welds are spaced from eachother in said discontinuous lines at intervals of from 4 to 15 mm. 15.An intermediate material according to claim 1 wherein said spot thermalbonding welds are in the form of an eyelet surrounding the perimeter ofeach of said holes that form said plurality of perforations through saidintermediate material.